The basic band-type compression and coding scheme for moving pictures includes predictive coding and transform coding. The high-efficiency coding is enabled utilizing known statistical characteristics of video signals, i.e., a correlation found existing between inter-picture pixels, and between intra-picture pixels. The predictive coding is utilizing the correlation in a time domain, and the transform coding is utilizing the correlation in a frequency domain.
The predictive coding is a scheme for coding a differential signal with a predictive picture derived by motion compensation prediction. The transform coding is a scheme for transforming pixel-based screen blocks into the frequency domain by Discrete Cosine Transform (DCT), and a transform coefficient of the resulting frequency components is quantized for transmission. Such a transform coefficient is hereinafter referred to as DCT coefficient. The recently-popular scheme is the combination of these two types of coding. For example, in ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) recommendation H.261 and H.263, or MPEG (Moving Picture Experts Group) as a working group of ISO (International Organization for Standardization) for picture compression, coding is performed for every 16-by-16 pixel block. Such a pixel block is hereinafter referred to as a macroblock.
In a moving picture coding device on the transmission end, coding is performed using motion compensation prediction, and thus a frame memory stores pictures being more or less different from original pictures. In this sense, after a predetermined length of time, the difference from the original pictures is increased, and eventually stored is picture data that is considerably different from the original pictures. If the resulting macroblock picture data is used for motion compensation of any neighboring blocks, the erroneous data is spread out in the neighborhood.
The prediction coding is a scheme for transmitting only a predictive error signal, and by accumulating the predictive error signal in a moving picture decoding device on the transmission-destination end, reproducing a picture signal. With such a configuration, if any coding error occurs in the transmission path, the erroneous picture data is input into a frame memory in the moving picture decoding device on the reception end. As a result, the original picture data stored in the frame memory is corrupted. Such picture corruption does not occur only to the original picture data in the frame memory but to any other subsequently incoming picture data because variable length data becomes out of sync. Therefore, in the moving picture decoding device, even if the subsequently incoming data is free from error, the correct picture data cannot be decoded and reproduced until the error effects are reset follows:
In consideration thereof, a conventional method has been proposed in which a differential value is derived between a predictive picture and a current picture for every macroblock. In the method, any macroblock with the differential value being a specific reference value or larger is refreshed by intra coding, and to any macroblock with the differential value being smaller than the reference value, inter coding is performed. In such a method, the macroblocks are refreshed every time any change of the reference value or more is observed in the picture motion so that picture quality deterioration can be prevented relatively in the early stages. As an example, refer to Patent Document 1 (JP-A-2001-169286; paragraphs of 0012 and 0013).
The problem here is that, with such conventional refreshing, if any transmission error is found in a specific macroblock during intra coding, the macroblock is not refreshed for a long time unless the picture in the macroblock is observed with any motion of the reference value or more. As a result, picture deterioration caused by the transmission error remains for a long period of time.
For the purpose of solving such a problem, another method is proposed in which every macroblock is unconditionally refreshed at regular intervals. The problem with this method is that intra coding is large in data amount for coding. Therefore, as described above, if refreshing is unconditionally performed to every macroblock, the coding efficiency is considerably reduced.